Dealkylation of alkylaromatic hydrocarbons

ABSTRACT

BE BUILT INTO THE DEALKYLATOR PRODUCT STILL. THE PRODUCT STILL IS MODIFIED TO WITHDRAW THE TOLUENE RECYCLE STREAM FROM A PLATE BETWEEN TWO REBOILERS AND THE TOLUENE FEED IS INTRODUCED TO THE STILL BELOW THE INLET FOR DEALKYLATOR PRODUCT.   PROCESS FOR PRODUCING BENZENE BY CATALYTIC HYDRODEALKYLATION OF TOLUENE. THE PROCESS IS PARTICULARLY ADAPTED TO ACCEPT, AS A FEEDSTOCK, RELATIVELY LOW CONCENTRATION TOLUENE, E.G., AROMATICS EXTRACT FROM CATALYTIC REFORMATE, COMPRISING 50-80 MOL PERCENT TOLUENE AND THE REMAINDER MAINLY BENZENE AND XYLENES. THE ADDIDITIONAL DISTILLATION CAPACITY TO SEPARATE BENZENE FROM THE TOLUENE FEED MAY

Jan. 26,1971 R. R. DE GRAF-F DEALKYLATION 0F ALKYLAROMATIC HYDRoAB'BoNsFiled May 16. 1968 United States Patent O 3,558,729 DEALKYLATION FALKYLAROMATIC HYDROCARBONS Richard R. De Graff, Arlington Heights, Ill.,assignor to Universal Oil Products Company, Des Plaines, Ill., a

corporation of Delaware Filed May 16, 1968, Ser. No. 729,719 Int. Cl.C07c 3/58; B01d 3/34 U.S. Cl. 260-672 8 Claims ABSTRACT OF THEDISCLOSURE BACKGROUND OF THE INVENTION This application relates to amethod for the dealkylation of alkyl-substituted aromatic hydrocarbons.More particularly, the invention relates to a method forhydrodealkylating alkyl-substituted aromatic hydrocarbons, such astoluene, in the presence of a catalytic composition of matter.

In recent years, the use of aromatic hydrocarbons, such as benzene, hasgreatly increased. For example, benzene of a high-grade purity is usedas an intermediate in the preparation of styrene monomer which may bepolymerized to form a type of rubber. Also, benzene is used as anintermediate in the preparation of phenol, aniline, maleic anhydride,insecticides such as DDT and benzene hexachloride,and as intermediatesin the preparation of synthetic fibers, such as some types of nylon.

Another relatively important use of benzene is as an intermediate in thepreparation of detergents, such as alkylaromatic sulfonates. Theselatter compounds are prepared by alkylating benzene with a long chainpolymer containing from about 12 to 15 carbon atoms or more in thechain. Following the alkylation of benzene, these long chainalkylbenzenes are then sulfonated by any method well known in the art toproduce the corresponding sulfonic acid. The acid may then beneutralized to form the corresponding sulfonate, such as the sodium orpotassium salt of the sulfonic alkylaromatic acid.

The dealkylation of alkylaromatic hydrocarbon is, generally, well knownin the art and may be performed either thermally or catalytically. Thereaction is exothermic in nature and must be carefully controlled inorder to prevent a runaway reaction, i.e. the reaction temperatureincreases beyond acceptable and desirable limits. Additionally, the mostefficient dealkylation reaction to produce benzene would be to supply asfeed relatively high purity toluene so that a minimum of side products,such as gaseous hydrocarbons and polymers, will be formed. However, incommercial practice, high purity toluene can only be obtainedeconomically from the reforming operation or coal tar operation havingassociated therewith a relatively expensive solvent extraction stepwhich separates the toluene from the other C8 aromatic hydrocarbonsproduced, ifor example, in a typical catalytic reforming step utilizingplatinum catalyst.

Accordingly, it would be desirable to have a method for producingbenzene via dealkylation of toluene which 3,558,729 Patented Jan. 26,1971 ICC can utilize a relatively low concentration toluene feedstock.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a method for the dealkylation of toluene to producebenzene.

It is another object of this invention to provide a method for producingbenzene via hydrodealkylation of toluene in a facile and economicalmanner.

It is a particular object of this invention to provide an improvedmethod for producing benzene from toluene wherein relatively lowconcentration toluene feedstocks may be utilized.

In the practice of the present invention there is provided a method forproducing benzene via dealkylation of toluene which comprises the stepsof: (a) introducing a feed stream comprising toluene into afractionation zone maintained under fractionation conditions at a locusintermediate the ends of said zone; (b) introducing a hereinafterspecified benzene-containing stream into said fractionation zone at alocus above said feed locus; (c) supplying heat for fractionation insaid fractionation zone through first reboiler means spaced below saidfeed locus; (d) supplying heat for fractionation in said fractionationzone through second reboiler means spaced below said rst reboiler means;(e) withdrawing from said fractionation zone an overhead distilalteproduct stream, a first side-cut benzene stream, a second side-cuttoluene stream, and a bottom stream comprising higher-boiling aromatichydrocarbons; (f) passing at least a portion of said second side-cutstream of step (e) into a dealkylation reaction zone maintained underdealkylation conditions; g) separating a benzene-containing stream fromthe effluent of said reaction zone; (h) passing said benzene stream ofstep (g) into said fractionation zone as specified in step (b); and (i)recovering benzene in high concentration from said fractionation zone.

Another embodiment of this invention includes the method hereinabovewherein said second side-cut fraction of step (e) is withdrawn from alocus between said first and second reboiler means.

In brief, therefore, the present invention provides improvement in thesimultaneous processing of the efuent from the reaction zone and arelatively impure feed stream utilizing a single fractionation zone. Byoperating in the manner specified, a relatively low concentrationtoluene feed stream may be processed through the system, in such amanner that the actual feed to the dealkylation reaction zone isrelatively concentrated thereby providing economy of operation andeconomy of capital investment in terms of reactor size, pump size,^etc.It should also be noted that the single fractionation zone is operatedby having at least two (2) reboiler systems, preferably, each locatedinternally to the fractionation column. The benets to be gained from thepresent invention reside in this dual reboiler feature and thewithdrawal of the relatively concentrated toluene stream from a locationwithin the fractionation zone between the two (2) reboiler means. Aswill become more evident from the description presented hereinbelow, inthe preferred embodiment of the invention, the upper reboiler meansutilizes relatively low pressure steam as the external heat media whilethe lower reboiler means utilizes relatively high pressure steam as theexternal heating media.

DETAILED DESCRIPTION OF THE INVENTION The method of this inventionutilizes the hydro-dealkylation reaction for removing the alkyl sidechains from the benzene neucleus. Therefore, the feedstock to thereaction zone should be a concentrate of toluene; although, to someextent, the xylenes may also be converted to benzene via dealkylation.However, the dealkylation reaction is not always selective in thatxylenes frequently are only reduced to toluene and/or the severity ofthe reaction is such that the benzene ring is opened or cracked therebydecreasing the aromatic hydrocarbon yield from the overall reaction.However, it is a critical feature of this invention that the feed streamcomprise relatively low concentration toluene, e.g. about 50% to 80%toluene on a mol basis with diluting constituents comprising mainlybenzene and the xylenes. As those skilled in the art will appreciatefrom the teachings presented herein, if relatively concentrated tolueneis available, there is very little commercial incentive for practicingthe method of the present invention. Preferably, the feed stream to thepresent invention will comprise from 70% to 80% toluene and, generally,will be obtained from a catalytic reforming operation.

Desirably, the dealkylation reaction of the present invention isperformed in the presence of a catalyst which may comprise a solublesalt of a metal of Group IV of the Periodic Table, such as, for example,tin chloride, titanium chloride, zirconium oxychloride, tin nitrate, tinsulfate, titanium nitrate, titanium sulfate, zirconium oxybromide,zirconium oxyiodide, etc., is coprecipitated with an alumina sol. Adetailed description of the suitable catalyst for the practice of thepresent invention may be found in U.S. Pat. No. 3,197,523 which issuedto Michalko et al. on July 27, 1965. However, it is within the conceptsof the present invention that the dealkylation reaction be etfectuatedthermally, i.e., in the substantial absence of catalytic materials.Those skilled in the art are familiar with the details of dealkylation;therefore, additional description of this well known reaction need notbe presented herein.

Suitable dealkylation reaction conditions utilizing the catalyticcomposite referred to hereinabove will include a temperature from 1,000F. to 1,500 F. and a pressure from 300 p.s.i.g. to 1,000 p.s.i.g.Desirably, a molar excess of hydrogen is also maintained in the reactionzone in order to reduce the possibility of catalyst deactivation due todeposition of coke or other heavy carbonaceous material upon thecatalytically active centers and surfaces of the catalyst. The liquidhourly space velocity through the catalyst bed may range from 0.5 to andthe hydrogen to hydrocarbon mol ratio may range from 3:1 to 15:1.Additionally, while the method of this invention may be effected ineither a batch type or continuous type operation, it is distinctlypreferred that a continuous operation be utilized.

It is to be noted that the fractionation zone of the present inventioninvolves, preferably, a single distillation column having a feed inletfor the dealkylated product stream, a feed inlet for the relatively lowconcentration toluene stream, a side-cut stream for withdrawing highpurity benzene, and a side-cut stream for withdrawing a relatively highpurity toluene. These distinctive features are accomplished through theunique technique of having two (2) reboiler means, preferably, locatedinternally within the fractionation zone. It is not intended to limitthe present invention to the use of any predetermined numbers of levelsof reboilers in as much as one, two, or more reboilers may be utilizedwithin the'column. Also, side reboilers or external reboilers may beused in combination With or instead of the internal reboilers wheredesired within the scope of this invention. In addition, various formsor designs of liquid wells to accommodate the nternal reboiler means maybe utilized as long as there is proper weir means to effect a liquidoverow from each zone containing a reboiler and a substantially constantliquid level in such zone to permit uniform heating over the reboilertubes.

Therefore, another embodiment of this invention includes the broadmethod hereinabove wherein said first and second reboiler means areinternal and a portion of said bottoms stream of step (e) is returned tosaid fractionation zone to provide a liquid seal over said secondreboiler means.

Operation of the fractionation zone in accordance with this invention,therefore, removes non-benzene components as an overhead distillatefraction and non-dealkylatable components as a bottoms residue fraction.By way of emphasis, the benefits which are gained from the practice ofthis invention involve the utilization of the fractionation zone asdescribed above operating in conjunction with a dealkylation reactionzone.

Subsequent to the obtaining of a relatively concentrationtoluene-containing stream from the fractionation zone, this concentrateof toluene is passed into the dealkylation reaction zone operating,preferably, in the presence of a molar excess of hydrogen and apreferred catalytic composite of the type described herein.

The eluent from the reaction zone is then, typically, passed throughheat exchange means into a separation zone wherein hydrogen gas forrecycle purposes is obtained and returned to the reaction zone. Theremaining liquid from the separation zone is then passed into strippingand/or absorption means wherein normally gaseous hydrocarbons areobtained and separated from the benzene-containing dealkylated productstream. This benzene-containing stream is withdrawn from the strippingand/or absorption zone and passed into a treating zone, such as withclay, wherein impurities which might impair the acid-wash color test forbenzene are removed. Finally, the clay treated product containingbenzene is passed into the fractionation zone at a locus generallyintermediate the ends of the fractionation zone and above the inletlocus for the relatively low concentration toluene feed stream which hadpreviously been introduced into such zone.

The ultimate benzene product is of relatively high purity in that,typically, it may have a 5.5 C. freeze point. Also, practice of thepresent invention will typically handle a toluene feed stream containingabout 70% toluene and produce a reaction zone feed stream containing atleast on a mol basis toluene.

The invention may be more fully understood with reference to theappended drawing which is a schematic representation of apparatus forpracticing one embodiment of the present invention.

DESCRIPTION OF THE DRAWING It is understood that the drawing as well asthe description thereof is presented herein for the sole purpose ofillustration and is not intended to limit the method of the presentinvention beyond the scope of the appended claims. In the interest ofsimplicity, various heaters, condensers, valves, control instruments,vessel configurations, etc. have been eliminated from the drawing,leaving only those vessels, connecting lines, and block representationswhich are necessary for a complete understanding of the process.

Referring now to the drawing, a relatively low concentration toluenestream diluted with other aromatic hydrocarbons and obtained as aby-product oil from a coke oven operation or from a catalytic reforimngoperation is introduced into the system via line 10 and passed intofractionation column 11 at a locus intermediate the ends offractionation column 11. A typical toluene-containing stream maycomprise on a commercial scale 154 mols per hour of toluene, 48 mols perhour of benzene, and 12 mols per hour of xylenes. This feed streamenters fractionator 11 at a temperature of about 250 F. and a pressureof about 9 p.s.i.g. Suitable fractionation conditions are maintained infractionator 11 whereby a light hydrocarbon stream comprising, forexample, cyclohexane, is withdrawn as an overhead distillate fractionvia line I13 at a temperature of about F. and a pressure of about 5p.s.i.g.

A hereinafter described benzene-containing stream comprising thehydrocarbon efiuent from the reaction zone is also introduced intofractianator 11 via line 12 at a locus above said feed locus for line10. This benzenecontaining stream on a commercial scale may comprise 159mols per hour of benzene, 70 mols per hour of toluene, 1.2 mols per hourof Xylene, 2.9 mols per hour of CNHI@ (diphenyl), 3.2 mols per hour ofCloHg (naphthalene), and 3.8 mols per hour of CmHm (anthracene) andenters fractionator 11 at a temperature of 220 F. and a pressure ofabout 8 p.s.i.g.

An upper side-cut fraction comprising high purity Ibenzene is withdrawnvia line 14 at a temperature of 195 F. and a pressure of about p.s.i.g.Typically, this upper side-cut stream will comprise about 207 mols perhour of 5 C. freeze point benzene.

A lower side-cut stream comprising relatively concentration toluene iswithdrawn via line 16 at a temperature of about 275 F. and a pressure ofabout 12 p.s.i.g. This lower side-cut stream on a commercial basis maycomprise 223 mols per hour of toluene, 14 mols per hour of xylene, 2.8mols per hour of ClgHm, 3 mols per hour of ClOHg, and 3.6 mols per hourof CMHm. It should be noted at this point that the material in line 16comprises about 90% on a mol basis toluene whereas the feed materialentering via line 10 comprised only about 72% toluene on a mol basis.

A bottoms stream comprising higher-boiling heavy hydrocarbons (eg.dicyclic aromatic hydrocarbons) is withdrawn from fractionator 11 vialine 15 at a temperature of 405 F. and a pressure of about 12 p.s.i.g.Preferably, the net amount of material in line 15 is withdrawn via line17 and sent to storage for use as fuel. If necessary or desirable, aportion of the material in line :15 may be passed via line 18 into line16 for further processing as more fully described hereinbelow.

In accordance with the preferred embodiment of the invention presentedhereinabove, fractionator 411 comprises at least two (2) internalreboiler means 23 and 24, respectively. Preferably, reboiler means 23 isheated by relatively low pressure steam (eg. 150 p.s.i.g.) which passesthrough reboiler 23 via lines 21. Suitable well means 25 havingassociated therewith a weir is also utilized in the column in order tomaintain a liquid seal over the tubes of reboiler 23.

Similarly, in the preferred embodiment, reboiler 24- is heated byrelatively high pressure steam (eg. 600 p.s.i.g.) which passes throughreboiler 24 via lines 22. Well 26 having associated therewith suitableWeir means is also a desirable part of fractionator 11 in order tomaintain a liquid seal over the tubes of lower reboiler means 24.

As a distinctly preferred manner of operation a cycle of higher-boilingheavy aromatic hydrocarbons is maintained via lines 1S and 20 incontinuous fashion in order to maintain a liquid seal over the tubes oflower reboiler means 24.

It is also to be noted that two of the essential features of thepreferred embodiment of this invention includes the withdrawal of thetoluene concentrate stream via line 16 at a locus between internalreboiler means 23 and 24. By operating in this manner, thehigher-boiling heavy aromatic hydrocarbons are rejected from the bottomof the column via line 15 and any toluene constituents therein arestripped out by the added heat which is introduced into the column viainternal reboiler 24. Similarly, any benzene which has passed down thecolumn into the vapor-liquid contacting means or trays normallyassociated with fractionating columns is also stripped out by the heatwhich is introduced into the column via internal reboiler 23.

Returning now to the toluene concentrate stream now comprising about 90%toluene on a mol basis: this material is now passed via line 19 whereinit is mixed with a hydrogen-containing gas stream from line 27 andpassed via line 28 into hydrodealkylation reaction zone 29 whichcontains, preferably, a dealkylation catalyst. Illustrative of thisoperation, the total reactor hydrocarbon charge may comprise 224 molsper hour of toluene, 13 mols per hour of xylene, 2.8 mols per hour ofCmHlo, 3.1 mols per hours of CIOHB, and 3.6 mols per hour of CMHIU.Therefore, with the presence of a molar excess of hydrogen gas from line27, the combined feed to the reactor in line 28 may comprise 1,776.14mols per hour of hydrogen, 1,053.53 mols per hour of methane, 80.12 molsper hour of ethane, 19.45 mols per hour of benzene, 226.68 mols per hourof toluene, 13.71 mols per hour of Xylene, 2.84 mols per hour of C12H10,3.07 mols per hour of C10H8, and 3.55 mols per hour of CMH, and entersreactor 29 at a temperature of 1,275 F. and a pressure of about 570p.s.1.g.

The effluent is withdrawn from reactor 29 via line 30 at a temperatureof about l,340 F. and is immediately quenched by a recycle stream fromline 33 to a temperature of about 1,000 F. Preferably, the quenchingzone is located internal the reactor shell. The quenched etiiuent isthen passed via line 30 into separator 31 which may comprise one or moreseparation vessels which are lmaintained under conditions sufficient toseparate a hydrogencontaining gas via line 27 which is returned to thereaction zone as previously discussed. As those skilled in the art arefamiliar, a make-up hydrogen stream from a source, not shown, may alsobe introduced into line 27. The separator liquid is withdrawn via line32 and a portion thereof returned via line 33 as quench to reactor 29 inthe manner previously discussed. The net a-mount of separator liquid inline 32 is passed into stripping zone 34 wherein suitable conditions aremaintained to produce overhead the normally gaseous materials which aresent via line 35, generally, to a refinery fuel system. The compositionof the material in line 35 on a commercial basis may comprise 1.6 molsper hour of hydrogen, 8.2 mols per hours of methane, 2.8 mols per hourof ethane, and .3 mol per hour of benzene. Operating conditions instripper 34 include an overhead temperature of about F. and an overheadpressure of about 218 p.s.i.g. with a bottoms temperature of about 430F. and a bottoms pressure of about 220 p.s.1.g.

The stripper bottoms material comprising a benzene concentrate iswithdrawn via line 36 and passed into conventional clay treating means37 for the re-moval therein of certain impurities which might impair theacid-wash color test of the ultimate benzene which is recovered inractionator 11 previously discussed. The clay treated benzene-containingstream is withdrawn via line 12 and passed into fractionator 11 in themanner previously discussed.

From the description of the invention presented thus far, it can be seenthat another embodiment of this invention includes the method forproducing benzene via hydrodealkylation of toluene which comprises thesteps of: (a) introducing a feed stream comprising relatively lowconcentration toluene diluted with other aromatic hydrocarbons into afractionation zone maintained under fractionation conditions at a locusintermediate the ends of said zone; (b) introducing a hereinafterspecified benzenecontaining stream into said fractionation zone at alocus above said feed locus; (c) withdrawing from said fractionationZone an overhead distillate product stream comprising relatively lighthydrocarbons, an upper side-cut stream comprising high purity benzene, alower side-cut stream comprising relatively concentration toluene, and abottoms stream comprising higher-boiling heavy hydrocarbons; (d) passingsaid lower side-cut stream into a hydrodealkylation reaction zonemaintained under conditions including the presence of hydrogen gassufficient to dealkylate toluene to benzene; (e) separating abenzenecontaining stream from the eflluent of said reaction zone; and(f) passing said benzene-containing stream into said fractionation zoneas speciiied in step (b).

The invention claimed:

1. Method for producing benzene via dealkylation of toluene whichcomprises the steps of:

(a) introducing a feed stream comprising from about 50 to 80 mol percenttoluene diluted with other aromatic hydrocarbons including benzene intoa unitary fractionation zone maintained under fractionation conditionsat a locus intermediate the ends of said zone;

(b) introducing a hereinafter specified benzene-containing stream intosaid fractionation zone at a locus above said feed locus;

(c) supplying heat for fractionation in said fractionation zone throughfirst reboiler means spaced below said feed locus;

(d) supplying heat for fractionation in said fractionation zone throughsecond reboiler means spaced be low said first reboiler means;

(e) withdrawing from said fractionation zone an overhead distillateproduct stream, a first side-cut benzene stream, a second side-cuttoluene stream containing a higher concentration of toluene than saidfeed stream, and a bottoms stream comprising higher boiling aromatichydrocarbons;

(f) passing at least a portion of said second side-cut stream of step(e) into a dealkylation reaction zone maintained under dealkylationconditions;

(g) separating from the effluent of said reaction zone a streamcontaining a higher concentration of benzene than said feed stream;

(h) passing said benzene stream of step (g) into said fractionation zoneas specified in step (b); and,

(i) recovering benzene in high concentration from said fractionationzone as said first side-cut stream.

2. Method according to claim 1 wherein said second side-cut fraction ofstep (e) is withdrawn from a locus between said first and secondreboiler means.

3. Method according to claim 2 wherein said first and second reboilermeans are internal and, a portion of said bottoms stream of step (e) isreturned to said fractionation zone to provide liquid seal over saidsecond reboiler means.

4. Method according to claim 2 wherein said first and second reboilermeans are heated by external heating media.

5. Method for producing benzene via hydrodealkylation of toluene whichcomprises the steps of (a) introducing a feed stream comprising fromabout 50 to 80 mol percent toluene diluted with other aromatichydrocarbons including benzene into a unitary fractionation zonemaintained under fractionation conditions at a locus intermediate theends of said zone;

(b) introducing a hereinafter specified benzene-containing stream intosaid fractionation zone at a locus above said feed locus;

(c) withdrawing from said fractionation zone an overhead distillateproduct stream comprising relatively light hydrocarbons, an upperside-cut stream c0mprising benzene concentrate, a lower side-cut toluenestream containing a higher concentration of toluene than said feedstream, and a bottoms stream comprising higher-boiling hydrocarbons;

(d) passing said lower side-cut stream into a hydrodealkylation reactionzone maintained under conditions including the presence of hydrogen gassufficient to dealkylate toluene to benzene;

(e) separating from the effluent of said reaction zone a streamcontaining a higher concentration of benzene than said feed stream; and,

(f) passing said benzene containing stream into said fractionation zoneas specified in step (b).

6. Method according to claim 5 wherein said reaction zone contains adealkylation catalyst.

7. Method according to claim 1 wherein said feed stream contains aboutto 80 mol percent toluene.

8. Method according to claim 5 wherein said feed stream contains about70 to 80 mol percent toluene.

References Cited UNITED STATES PATENTS 3,197,523 7/1965 Michalko et al260-672 3,304,340 2/ 1967 Noll 260-672 3,310,593 3/1967 Nelson et al260-672 3,374,280 3/1968 Carr et al. 260-672 3,435,084 3/1969 Cabbage260-667 3,477,915 11/1969 Gantt et al. 202,-155

DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant ExaminerU.S. C1. X.R.

